Effective meso Fabrications of Subporphyrins

The First Synthesis of Subporphyrins

Because of the rich coordination chemistry and unique optical and electrochemical properties, porphyrin analogues have been intensively investigated. Among them, subporphyrins have been long-awaited molecules, with only their boron complexes known to date because of the crucial role of the central boron atom as a template in synthesis. The challenges related to the synthesis of a genuine subporphyrin (boron-free) have finally been met and reported in a recent article from Kim, Osuka, and Song. A key strategy of their synthesis lies in the introduction of an exocyclic double bond at the meso-position and subsequent reduction to obtain macrocyclic conjugation. Considering the fact that the development of porphyrin chemistry is ultimately linked to the availability of free base porphyrins, this seminal work will facilitate studies on coordination chemistry and applications in materials science.

10,15-Bis(ethoxycarbonyl)-5-(4-methoxycarbonylphenyl) B(III)subchlorin: A photosensitizer with high singlet oxygen producing efficiency

A novel A2B-type B(III)subchlorin has been synthesized for the first time in two ways possessing two different ester moieties upon macrocyclic periphery from meso-diethoxycarbonyl tripyrrane. Its photophysical and electrochemical properties have been explored. Introduction of the third meso-substituent resulted in the synthesis of the B(III)subchlorin as the major product with the formation of minor oxidized B(III)subporphyrin analogue. This subchlorin derivative was found to generate singlet oxygen much efficiently with quantum yield ([Formula: see text] 0.88.

Nucleophilic Aromatic Substitution (SNAr) and Related Reactions of Porphyrinoids: Mechanistic and Regiochemical Aspects

The nucleophilic substitution of aromatic moieties (SNAr) has been known for over 150 years and found wide use for the functionalization of (hetero)aromatic systems. Currently, several "types" of SNAr reactions have been established and notably the area of porphyrinoid macrocycles has seen many uses thereof. Herein, we detail the SNAr reactions of seven types of porphyrinoids with differing number and type of pyrrole units: subporphyrins, norcorroles, corroles, porphyrins, azuliporphyrins, N-confused porphyrins, and phthalocyanines. For each we analyze the substitution dependent upon: a) the type of nucleophile and b) the site of substitution (α, β, or meso). Along with this we evaluate this route as a synthetic strategy for the generation of unsymmetrical porphyrinoids. Distinct trends can be identified for each type of porphyrinoid discussed, regardless of nucleophile. The use of nucleophilic substitution on porphyrinoids is found to often be a cost-effective procedure with the ability to yield complex substituent patterns, which can be conducted in non-anhydrous solvents with easily accessible simple porphyrinoids.

meso-Free BIII Subporphyrins with Electron-donating Groups

meso-Free B^III 5,10-bis(p-dimethylaminophenyl)subporphyrins were synthesized. They display red-shifted absorption and fluorescence spectra, bathochromic behaviors in polar solvents, a high fluorescence quantum yield (Φ_F =0.57), and a small HOMO-LUMO gap mainly due to destabilized HOMO as compared with meso-free B^III 5,10-diphenylsubporphyrin. This subporphyrin serves as a nice precursor of various meso-substituted B^III subporphyrins such as B^III meso-nitrosubporphyrin, B^III meso-aminosubporphyrin, and meso-meso' linked B^III azosubporphyrin dimer. Reactions of meso-free B^III subporphyrins with NBS or bis(2,4,6-trimethylpyridine)bromonium hexafluorophosphate gave meso-meso' linked subporphyrin dimers, often as a major product along with meso-bromosubporphyrins.

Singly and Doubly Quinoxaline-Fused BIII Subporphyrins

B-Phenyl B^III subporphyrin-α-diones prepared in a three-step reaction sequence from the parent subporphyrin were condensed with 1,2-diaminobenzenes to give the corresponding quinoxaline-fused subporphyrins in variable yields. Quinoxaline-fused B-phenyl-5,10,15-triphenyl B^III subporphyrin was transformed to the corresponding subporphyrin-α-dione in the same three-step reaction sequence, which was then condensed with 1,2-diaminobenzene to give doubly quinoxaline-fused subporphyrin. These quinoxaline-fused subporphyrins exhibit redshifted absorption and fluorescence spectra compared with the parent one. A singly quinoxaline-fused subporphyrin bearing three meso-bis(4-dimethylaminophenyl)aminophenyl substituents shows blueshifted fluorescence in less polar solvent, which has been ascribed to emission associated with charge recombination of intramolecular charge transfer (CT) state.

Acetylene and trans-Ethylene Bridged BIII -Subporphyrin Dimers

Acetylene and trans-ethylene bridged B^III -subporphyrin dimers were synthesized by cross-coupling reactions of meso-bromo B^III subporphyrin. These dimers display perturbed and red-shifted absorption spectra reaching around 750 nm and fluorescence reaching at around 850 nm with high quantum yields of 0.39 and 0.47, respectively. DFT calculations have revealed that the HOMOs and the LUMOs of both dimers are spread over the two subporphyrin units as an indication of effective conjugation between the two subporphyrin units. The large Stokes shifts and characteristic pico-second time-resolved transient absorption spectra indicated that the S₁ -states of the dimers relax with structural changes, which are larger for the trans-ethylene bridged dimer.

BIII 5-Arylsubporphyrins and BIII Subporphine

Boron(III) 5-arylsubporphyrins and B^III subporphine are promising precursors for functional B^III subporphyrins bearing asymmetric meso-substituents. Herein, we report the first synthesis of these molecules. Among many aryl acid chlorides examined, 4-nitrobenzoyl chloride gave B^III 5-(4-nitrophenyl)subporphyrin in 10 % yield in condensation with triethylamine-tri-N-tripyrromethene-borane. The nitro group of this B^III subporphyrin was reduced with NaBH₄ to prepare B^III 5-(aminophenyl)subporphyrin, which was converted into B^III 5-phenylsubporphyrin via the corresponding diazonium salt. B^III subporphine was synthesized by condensation of triethyl orthoformate with triethylamine-tri-N-tripyrromethene-borane. Progressive removal of meso-phenyl substituents leads to continuous changes in the optical properties, whereas the B^III subporphine deviates from this trend in some properties.

meso-Functionalization of Boron(III) Subporphyrin with Boron(III) meso-Lithiosubporphyrin

Boron(III) meso-lithiosubporphyrin was prepared by bromine-lithium exchange of B-tolyl B^III meso-bromosubporphyrin with n-butyllithium at -98 °C. The resulting subporphyrinyllithium was treated with various electrophiles such as benzophenone, N,N-dimethylformamide, CO₂ , chlorotrimethylsilane, N-fluorobenzenesulfonimide, and dimesitylboryl fluoride to give the corresponding meso-functionalized B^III subporphyrins. The nucleophile was also used to construct B^III subporphyrin dimers such as bis(B^III subporphyrinyl)ketone, bis(B^III subporphyrinyl)carbinol, and disilane-bridged B^III subporphyrin dimer. The structural, optical, and electrochemical properties of these meso-functionalized B^III subporphyrins were examined by UV/Vis absorption and fluorescence spectroscopy, electrochemical studies, and DFT calculations.

Synthesis of Boron(III)-Coordinated Subchlorophins and Their Peripheral Modifications

A pyrrole-cleaving modification to transform boron(III) meso-triphenylsubporphyrin into boron(III) meso-triphenylsubchlorophin has been developed. Boron(III) subchlorophins thus synthesized show absorption and fluorescence spectra that are roughly similar to those of boron(III) subchlorins, but B-methoxy boron(III) subchlorophin showed considerably intensified fluorescence and a small Stokes shift. Peripheral modification reactions of B-phenyl boron(III) subchlorophin such as regioselective nitration with Cu(NO₃ )₂ ⋅3 H₂ O, ipso-substitution reactions of boron(III) α-nitrosubchlorophin with CsF and CsCl, and Pd-catalyzed cross-coupling reactions of boron(III) α-chlorosubchlorophin with arylacetylenes, have been also explored to tune the optical properties of subchlorophins.

meso-Nitro- and meso-Aminosubporphyrinatoboron(III)s and meso-to-meso Azosubporphyrinatoboron(III)s

meso-Nitrosubporphyrinatoboron(III) was synthesized by nitration of meso-free subporphyrin with AgNO₂ /I₂ . The subsequent reduction with a combination of NaBH₄ and Pd/C gave meso-aminosubporphyrinatoboron(III). meso-Nitro- and meso-amino-groups significantly influenced the electronic properties of subporphyrin, which has been confirmed by NMR and UV/Vis spectra, electrochemical analysis, and DFT calculations. Oxidation of meso-aminosubporphyrinatoboron(III)s with PbO₂ cleanly gave meso-to-meso azosubporphyrinatoboron(III)s that exhibited almost coplanar conformations and large electronic interaction through the azo-bridge.

Nucleophilic aromatic substitution reactions of meso-bromosubporphyrin: synthesis of a thiopyrane-fused subporphyrin

meso-Bromosubporphyrin undergoes nucleophilic aromatic substitution (SN Ar) reactions with arylamines, diarylamines, phenols, ethanol, thiophenols, and n-butanethiol in the presence of suitable bases to provide the corresponding substitution products. The SN Ar reactions also proceed well with pyrrole, indole, and carbazole to provide substitution products in moderate to good yields. Finally, the SN Ar reaction with 2-bromothiophenol and subsequent intramolecular peripheral arylation reaction affords a thiopyrane-fused subporphyrin.

Probing the rotational dynamics of meso-(2-substituted)aryl substituents in A2B-type subporphyrins

A2 B-type B-methoxy subporphyrins 3 a-g and B-phenyl subporphyrins 7 a-c,e,g bearing meso-(2-substituted)aryl substituents are synthesized, and their rotational dynamics are examined through variable-temperature (VT) (1) H NMR spectroscopy. In these subporphyrins, the rotation of meso-aryl substituents is hindered by a rationally installed 2-substituent. The rotational barriers determined are considerably smaller than those reported previously for porphyrins. Comparison of the rotation activation parameters reveals a variable contribution of ΔH(≠) and ΔS(≠) in ΔG(≠). 2-Methyl and 2-ethyl groups of the meso-aryl substituents in subporphyrins 3 e, 3 f, and 7 e induce larger rotational barriers than 2-alkoxyl substituents. The rotational barriers of 3 g and 7 g are reduced by the presence of the 4-dibenzylamino group owing to its ability to stabilize the coplanar rotation transition state electronically. The smaller rotational barriers found for B-phenyl subporphyrins than for B-methoxy subporphyrins indicate a negligible contribution of SN 1-type heterolysis in the rotation of meso-aryl substituents.

Subporphyrins with an axial B-C bond

Axial fabrications of subporphyrins have been conveniently accomplished by the reaction of B(methoxo)triphenylsubporphyrin with Grignard reagents such as aryl-, heteroaryl-, ferrocenyl-, β-styryl-, phenylethynyl-, and ethylmagnesium bromides. The axial groups thus introduced are not conjugated with the subporphyrin core. This situation leads to effective fluorescence quenching of subporphyrins when the axial group is strongly electron donating such as 4-dimethylaminophenyl and ferrocenyl groups.